8 replies to this topic

[TxT]

Hi there,

I'll make it short: I'm doing a project for "My story". That project is an industrial unit for the production of nonene through propylene.

In "My story", that propylene is obtained by a nearby refinery, that produces 80% purity propylene, in the FCC unit (I know that the temperature of the distillation outlet stream with propylene is 150ºC - although, I dunno the pressure).

With that said, which are the typical conditions of storage of propylene - can I store it at 150ºC?

Thanks in advance
Regards

[pavanayi]

TxT,
Storing propylene at 150°C would be a very bad idea. I would want to store it in liquid phase to have minimum storage capacities, and considering the fact that propylene has a critical temperature of 91°C, you would not be able to liquify it at that temperatures.

Depending on the cooling water temperature around your area, you may select a pressurised storage (sphere/bullet) after liquifying it. For example, 14~15 barg at 30°C is very much suitable in most locations for pure propylene. It can vary depending on what is the other 20% in your case.

[kkala]

I agree with pavanayi, adding that in an ethylene plant revamp study (1990) propylene was produced and stored in spheres. Design pressure corresponded to max ambient temperature plus margins dictated by applicable practice (say 10% on gauge pressure).
You could also investigate the option of bullets, each one limited to approx max capacity of 200 m3 (according to data of 1999, not expected to have changed).

Edited by kkala, 17 November 2011 - 05:38 AM.

[breizh]

Hi ,
It might be interesting to read this document from lyondell :

http://www.lyondellb...echlit/3303.pdf

Breizh

[TxT]

The other 20% is propane. It's unusual to store propylene in gas phase?

Best regards

[pavanayi]

TxT,
It is unusual to store anything in gas phase if it can be liquefied cost effectively.
1ton of saturated 80/20 propylene/propane mixture at 15 barg will occupy approx 2 m³ in liquid phase and approx 28.6 m³ in gaseous phase.

Cost effectiveness comes in the factor of increased cost of storage for vapour phase against increased cost of pressurisation/condensation for liquid phase.

[TxT]

But that option is only related to the volume of the tank right? Or involves somekind of hazards?

[kkala]

--Pavanayi has pointed out the great difference in gas / liquid propylene volume per unit mass, which supports its storage in liquefied form, like other similar gases (propane, butane). This difference is shown in attached "propylene.xls" in function of its temperature, being more or less the ambient temperature (storage is neither heated nor cooled). Reference to propylene.xls is continually made below.
--Volume per unit mass, shown in the attachment, corresponds to saturation pressure. Suppose that we are sending 1000 kg of propylene gas to a storage of 37.8 m3, keeping constant temperature t=30 oC. The gas will not be condensed, but reach the saturation pressure of 13.2 bara. Gas in excess of 1000 kg will be totally condensed, but let us consider that this storage contains 1000 kg of propylene gas.
For temperatures higher than 30 oC, stored fluid will remain gas with pressure higher than 13.2 bara. For temperatures lower than 30 oC, stored propylene will be partially condensed, its pressure getting lower than 13.2 bara. Liquid phase will be settled at the bottom. In the ambient temperature range of 0 – 40 oC (assumed), propylene has to be drawn off sometimes as liquid, sometimes as gas, which is a drawback.
Now consider introduction of 1000 kg of propylene liquid into a storage of 2.1 m3. Stored fluid for all temperatures up to 40 oC (assumed maximum ambient) will be liquid, drawn off when needed from storage bottom. Of course storage will contain some gas over the liquid (or only gas, when emptied from liquid); but mass of gas will not be significant, less than 34 kg propylene per m3 of storage. This storage ensures liquid phase in whole range of ambient temperatures under normal operating conditions. Besides it is much smaller.
--Alternatively one could store liquid propylene at atmospheric pressure, keeping it at –48 oC through refrigeration. But this is economically justified only for big capacities, probably 8000 m3 or higher ( http://www.cheresour...h__1#entry39160). Semi refrigerated storage is an intermediate case, sometimes applied.
--Propylene temperature could be artificially kept high to ensure gaseous state at any possible operating pressure (condensation not possible at any pressure, if t > 92.5 oC). Apparently this is not economical due to heating.
--It is noted that above considers storage of pure propylene, without usual margins on capacity or pressure, for clarity and simplicity. In data sheet preparation care is needed to conservatively consider the vapor pressure of real fluid, according to its specifications (not e.g. merely considering 80% propylene and 20% propane, mol/mol).
--Since propylene gas is heavier than air (spreading over the soil surface, not lifted upwards by buoyancy) it can be more dangerous in a loss of containment event, compared to propylene liquid. The latter can slowly evaporate in the air after a sudden initial flashing. Additional advice to this opinion would be welcomed.

Attached Files

•  propylene.xls   36KB   150 downloads

[TxT]

kkala, thank you very much for your explanation, it was very helpful.